The present invention relates to a method of treatment of a high-level radioactive waste generated, for example, from reprocessing of spent nuclear fuels. More particularly, the present invention is concerned with a method for treating a high-level radioactive waste which comprises adding a suitable amount of boron or a boron compound to a calcined material of the high-level radioactive waste, treating the resultant mixture at a high temperature to alloy platinum group elements contained in the waste with boron, separating and recovering the resultant alloys, and solidifying residual oxides as a solid waste of a high degree of volume reduction.
A high-level radioactive waste generated from reprocessing of spent fuels by purex process is stored in the form of a nitric acid solution containing fission products. This liquid waste is solidified in the future through inclusion in a medium such as glass. Besides glass, many materials such as synthetic rock and the like have been studied as the medium. The concentration of the fission products in the medium is limited to about 10% by weight from the viewpoint of problems such as the solubility of the fission products into the medium, chemical durability (leaching rate in water), and removal of decay heat. The volume of the solidified waste should be as small as possible for the purpose of decreasing the cost of storage and disposal thereof. Although the fission products content of the solidified waste must be increased for this purpose, it is difficult at the present time due to the reasons described above.
Meanwhile, the high-level radioactive waste contains platinum group elements (Ru, Pd and Rh) which are useful but poor in natural resources. Various attempts have been made to recover these elements, and examples of the known method include:
(1) a solvent extraction method wherein these elements are separated from a nitric acid solution of the highd-level radioactive waste by using a phosphoric ester; PA1 (2) a lead extraction method wherein the high-level radioactive waste is vitrified and these elements are extracted from the vitrified waste by using molten lead; and PA1 (3) an ion-exchange method wherein these elements are separated by treating a nitric acid solution of the radioactive waste with an ion-exchange resin. PA1 (1) In the solvent extraction method, the phosphoric ester becomes a secondary waste which is different from the solvent for extraction in the reprocessing, i.e. TBP (tributyl phosphite). This makes it necessary to conduct research and development on a processing method and construction of a processing plant different from those of the waste TBP. The cost necessary for this purpose is very high and causes the cost of the recovery of the platinum group elements to be increased over that of the commercially available platinum group elements, so that the conventional solvent extraction method does not economically pay. PA1 (2) The lead extraction method is advantageous in that lead which becomes a solid waste as it is is used as the extractant. In this method, however, in order to enhance the extraction efficiency, it is necessary to use a low-viscosity glass having a composition different from that of the glass used for the vitrification of the high-level radioactive waste. Further, lead and the platinum group elements should be re-separated, thus rendering this method difficultly applicable to practical use. PA1 (3) The ion exchange method has a problem of safety because a flammable substance is formed when the ion-exchange resin comes into contact with nitric acid.
However, these prior art methods of recovering platinum group elements have the following drawbacks.
A large amount of a secondary waste occurs in any of the above-described prior art methods, so that a treatment for remarkably reducing the volume of the high-level radioactive waste cannot be accomplished.